Passive Vibration Control for Fatigue Damage Mitigation in Steel Jacket Platforms

Considering the stress cycles in the joints and members due to wave induced forces on offshore platforms, fatigue analysis is therefore one of the most important analyses in the offshore platforms design. Although most of the steel jacket type platforms are designed and located in areas with relatively high ratios of operational sea-states, for maximum environmental events, would have acceptable safety margin in an in-place and seismic analyses; but for fatigue analyses it faces critical condition. Therefore it seems utilizing control mechanisms with the aim of increasing fatigue life in such platforms will be more preferable, to merely deck displacement control. Investigation of tuned mass damper parameters optimality for vibration control of wave excited systems, implies that optimum tuning and damping ratios are strongly dependent on sea-state, in addition to system parameters. The efficiency of optimally designed tuned mass damper for fatigue damage mitigation in real steel jacket platforms has been evaluated using full stochastic spectral analysis method and the results have shown a great performance for TMDs in this application. Sea-state conditions are predictable through weather forecasting methods and this can be benefited also to increase the efficiency of TMDs with variable tuning parameters which have been optimally adjusted for each sea-state. In a case study, utilization of this auxiliary device resulted in 26.6 % reduction in maximum fatigue damage. This efficiency can be increased to 36.7 % using different tuning parameters for each sea-state.